5 For there are countless numbers who not only boast that they see, but would have it appear that they are enlightened by Christ; yet are they heretics. Have even they somehow entered by the gate? Surely not. Sabellius says, He who is the Son is Himself the Father; but if the Son, then is there no Father. He enters not by the door, who asserts that the Son is the Father. Arius says, The Father is one thing, the Son is another thing. He would say rightly if he said, Another person; but not another thing. For when he says, Another thing, he contradicts Him who says in his hearing, “I and my Father are One.” Neither does he therefore enter by the door; for he preaches a Christ such as he fabricates for himself, not such as the truth declares Him.
You have the name, you have not the reality. Christ is the name of something; keep hold of the thing itself, if you would benefit by the name. Another, I know not from whence, says with Photinus, Christ is mere man; He is not God. He enters not in by the door, for Christ is both man and God. But why need I make many references, and enumerate the many vanities of heretics? Keep hold of this, that Christ's sheepfold is the Catholic Church. Whoever would enter the sheepfold, let him enter by the door, let him preach the true Christ.
Not only let him preach the true Christ, but seek Christ's glory, not his own; for many, by seeking their own glory, have scattered Christ's sheep, instead of gathering them. For Christ the Lord is a low gateway: he who enters by this gateway must humble himself, that he may be able to enter with head unharmed. But he that humbles not, but exalts himself, wishes to climb over the wall; and he that climbs over the wall, is exalted only to fall.
Source: Tractates on the Gospel of John (New Advent)